1. Field of the Invention
The invention relates generally to the field of marine seismic survey apparatus and methods. More specifically, the invention relates to structures for marine seismic streamers that have reduced noise induced by effects of towing such streamers in the water.
2. Background Art
In a marine seismic survey, a seismic vessel travels on the surface of a body of water such as a lake or the ocean. The seismic vessel typically contains seismic acquisition control equipment, which includes devices such as navigation control, seismic source control, seismic sensor control, and signal recording devices. The seismic acquisition control equipment causes a seismic source towed in the body of water, by the seismic vessel or another vessel, to actuate at selected times. The seismic source may be any type well known in the art of seismic acquisition, including air guns or water guns, or most commonly, arrays of air guns. Seismic streamers, also called seismic cables, are elongate cable-like structures that are towed in the body of water by the seismic survey vessel or by another vessel. Typically, a plurality of seismic streamers is towed behind the seismic vessel laterally spaced apart from each other. The seismic streamers contain sensors to detect the seismic wavefields initiated by the seismic source and reflected from acoustic impedance boundaries in the subsurface Earth formations below the water bottom.
Conventionally, seismic streamers contain pressure-responsive sensors such as hydrophones, but seismic streamers have also been proposed that contain particle motion sensors, such as geophones, in addition to hydrophones. The sensors are typically located at regular intervals along the length of seismic streamers.
Seismic streamers also include electronic components, electrical wiring and may include other types of sensors. Seismic streamers are typically assembled from sections, each section being approximately 75 meters in length. A number of such sections are joined end to end, and can extend the assembled streamer to a total length of many thousands of meters. Position control devices, such as depth controllers, paravanes, and tail buoys are affixed to the streamer at selected positions and are used to regulate and monitor the movement of the streamer in the water. During operation, the seismic sources and streamers are typically submerged at a selected depth in the water. The seismic sources are typically operated at a depth of 5-15 meters below the water surface and the seismic streamers are typically operated at a depth of 5-40 meters.
A typical streamer section consists of an external jacket, connectors, spacers, and strength members. The external jacket is formed from a flexible, acoustically transparent material such as polyurethane and protects the interior of the streamer section from water intrusion. The connectors are disposed at the ends of each streamer section and link the section mechanically, electrically and/or optically to adjacent streamer sections and, therefore, ultimately link it to the seismic towing vessel. There is at least one, and are usually two or more such strength members in each streamer section that extend the length of each streamer section from one end connector to the other. The strength members provide the streamer section with the capability to carry axial mechanical load. A wire bundle also extends the length of each streamer section, and can contain electrical power conductors and electrical data communication wires. In some instances, optical fibers for signal communication are included in the wire bundle. Hydrophones or groups of hydrophones are located within the streamer section. The hydrophones have sometimes been located within corresponding spacers for protection. The distance between spacers is ordinarily about 0.7 meters. A hydrophone group, typically comprising 16 hydrophones, thus extends for a length of about 12.5 meters.
The interior of the seismic streamers is filled with a void filling material to provide buoyancy and desired acoustic properties. Most seismic streamers have been filled with a liquid core material, such as oil or kerosene. Such liquid-filled streamer design is well proven and has been used in the industry for a long time. However, there are disadvantages associated with using liquid as a core fill material. The first disadvantage is leakage of the liquid into the surrounding water in the event a streamer section is damaged. Such leakage self-evidently presents a serious environmental problem. Damage can occur while the streamer is being towed through the water or it can occur while the streamer is being deployed from or retrieved onto a streamer winch on which streamers are typically stored on the seismic vessel.
A second disadvantage to using liquid-filled streamer sections is noise induced in the hydrophones generated by vibrations as the streamer is towed through the water. Such vibrations develop internal pressure waves that travel through the liquid in the streamer sections, such waves often referred to as “bulge waves” or “breathing waves.” The foregoing noise is described, for example, in S. P. Beerens et al., Flow Noise Analysis of Towed Sonar Arrays, UDT 99—Conference Proceedings Undersea Defense Technology, Jun. 29-Jul. 1, 1999, Nice, France, Nexus Media Limited, Swanley, Kent.
Ideally, in a streamer moving at constant speed, all the streamer components including the jacket, the connectors, the spacers, the strength members, wire bundle, sensors and liquid void filling material all move at the same constant speed and do not move relative to each other. Under actual movement conditions, however, transient motion of the streamers takes place, such transient motion being caused by events such as pitching and heaving of the seismic vessel, movement of the paravanes and tail buoys attached to the streamers, strumming of the towing cables attached to the streamers caused by vortex shedding on the cables, and operation of depth-control devices located on the streamers. Any of the foregoing types of transient motion can cause transient motion (stretching) of the strength members.
Transient motion of the strength members displaces the spacers or connectors, causing pressure fluctuations in the liquid void filling material that are detected by the hydrophones. Pressure fluctuations radiating away from the spacers or connectors also cause the flexible outer jacket to compress in and bulge out in the form of a traveling wave, giving the phenomenon “bulge waves” its name.
In addition, there are other types of noise, often called “flow noise”, which can affect the quality of the seismic signal detected by the hydrophones. For example, vibrations of the seismic streamer can cause extensional waves in the outer jacket and resonance transients traveling down the strength members. A turbulent boundary layer created around the outer jacket of the streamer by the act of towing the streamer can also cause pressure fluctuations in the liquid core-filling material. In liquid filled streamer sections, the extensional waves, resonance transients, and turbulence-induced noise are typically much smaller in amplitude than the bulge waves, however they do exist and affect the quality of the seismic signals detected by the hydrophones. Bulge waves are usually the largest source of vibration noise because these waves travel in the liquid core material filling the streamer sections and thus act directly on the hydrophones.
Several concepts have been proposed to reduce such noise in steamer sections. For example, it is known in the art to introduce compartment blocks in liquid-filled streamer sections to stop bulge waves from traveling continuously along the entire length of the streamer. It is also known in the art to introduce open cell foam into the interior of the streamer section. The open cell foam restricts the flow of the liquid void filling material in response to transient-motion induced pressure changes and causes the energy to be dissipated into the outer jacket and the foam over a shorter axial distance. Another approach known in the art to address such noise is to combine several hydrophones into a series-connected group to attenuate the effects of a slow moving wave on the detected seismic signal. Typically, such approach is implemented by positioning an equal number of series connected hydrophones between or on both sides of selected spacers so that pairs of hydrophones sense equal and opposite pressure changes. Summing the hydrophone signals from such a group can then substantially cancel such noise.
Another approach to eliminating bulge waves is to eliminate the liquid from the interior of streamer sections, so that no medium exists in which bulge waves can develop. This approach is exemplified by the use of so-called solid streamers, using streamer sections filled with a solid core material. However, in any type of solid material, some shear waves will develop, which can increase the noise detected by the hydrophones. Shear waves cannot develop in liquid filled streamers because liquids have no shear modulus. Additionally, many conventional solid core materials are not acoustically transparent to the pressure waves that the hydrophones are intended to detect.
Another approach to the noise problem is to replace the liquid core material in a streamer section with a soft, flexible solid core material, such as gel. The introduction of a softer, flexible solid material may block the development of bulge waves compared to a liquid core material. A soft, flexible solid material may also attenuate the transmission of shear waves as compared to a harder material. However, there can still be a substantial transmission of shear waves through such soft, flexible solid material.
Using a soft, flexible material will eliminate a substantial portion of the problem with “bulge waves”, but the so-called Poisson effect from the strength members can increase. Because of the relatively high tensile stiffness of the strength members, transients generally travel along the strength members at velocities near to or greater than that of the sound velocity in water, such velocities typically in the range of 1000 to 1500 meters per second. The actual velocity of transients along the strength members depends mainly on the elastic modulus of the strength member material and the tension applied to the streamer as it is towed in the water. The lower the elastic modulus the more compliant the streamer will be, and thus the more transient energy it will dissipate as heat and the less will pass through the strength member. Special elastic sections are normally placed at either end of a streamer cable to reduce the effects of transients.
A streamer traveling through the water may be considered to have an inertial mass represented by M that is subject to viscous damping represented by c. If the spring constant of the elastic sections is k, then the simplified transfer function of the elastic sections can be derived by solving the equation of motion as:
                    h        =                  1                                                                      (                                      1                    -                                                                  ω                        2                                                                    ω                        n                        2                                                                              )                                2                            +                                                (                                      2                    ⁢                    δ                    ⁢                                          ω                                              ω                        n                                                                              )                                2                                                                        (        1        )            
where ωn=√(k/M), and δ=c/2ωn This transfer function has the form of a mechanical high-cut filter above the resonant frequency of ω=ωn. The elastic modulus of any type of section also determines the transient wave velocity. The transient wave velocity may be represented by the expression:C=√{square root over (E/M)}  (2)
where C is the velocity that a transient will travel in the stress member, E is the elastic modulus, and M is the mass per unit length. For any particular stress member material, the stiffness will normally increase with strength more than the mass per unit length and the velocity will also increase. Knowledge of this velocity may be useful in formulating the design of the streamer hydrophone array for noise rejection.
A related property is the mechanical impedance. The impedance, Z, may be determined by the expression:Z=√{square root over (E*M)}  (3)
Changes in the impedance may affect the relative degree of propagation and reflection of transient waves along the streamer.
There is still a need to further improve the attenuation of longitudinal waves transmitted through the strength members of marine seismic streamers.